1. Field of the Invention
The present invention relates to a loudspeaker diaphragm for use in a loudspeaker for reproducing sound in a portable electronic device such as a portable telephone or a portable game device, and a loudspeaker using such a loudspeaker diaphragm.
2. Description of the Related Art
Some small loudspeakers employ a loudspeaker diaphragm being a film member of a resin such as PEI (polyetherimide), wherein the film member is a single-piece member including a dome portion and an edge portion, the edge portion extending along the outer periphery of the dome portion. Where such a loudspeaker is an electrodynamic loudspeaker, a voice coil to which an audio signal current is supplied may be attached to the junction between the dome portion and the edge portion of the loudspeaker diaphragm on the rear surface of the loudspeaker diaphragm. The outer periphery of the edge portion of the loudspeaker diaphragm is fixed to a frame that is coupled to a small and light-weighted magnetic circuit, with the voice coil being disposed in a magnetic gap of the magnetic circuit. The shape of a loudspeaker diaphragm of a small loudspeaker influences the quality of the sound reproduced from the small loudspeaker and the overall dimensions of the small loudspeaker.
In some conventional small loudspeakers for use in portable game devices, or the like, not only the overall height of the magnetic circuit, but also that of the loudspeaker diaphragm, are reduced in an attempt to further reduce the size, the thickness and the weight. For example, Japanese Laid-Open Patent Publication No. 2005-184588 discloses a loudspeaker having a loudspeaker diaphragm whose cross section inside the voice coil has a generally dome-like shape, wherein a depression is formed in a portion of the generally dome-like shape in order to reduce the unwanted resonance and to reduce the overall height dimension of the loudspeaker diaphragm. Japanese Laid-Open Patent Publication No. 8-140183 discloses a diaphragm including a main dome portion and a sub-dome portion being continuous with, and extending around, the main dome portion, wherein the sub-dome portion includes a number of radial grooves extending so that the proximal end thereof reaches the boundary between the main dome portion and the sub-dome portion, in order to flatten the relative sensitivity over the range of 4 to 8 kHz.
Japanese Laid-Open Patent Publication No. 9-135491 discloses a loudspeaker having a diaphragm, including a dome portion, an edge portion, and a flat portion at the boundary between the dome portion and the edge portion, wherein reinforcement ribs are formed in the rising portion of the dome portion along the boundary and in the rising portion of the edge portion along the boundary so as to increase the rigidity in the radial direction in the junction portion where a voice coil is attached to the diaphragm, in order to suppress abnormal vibrations of the diaphragm due to the natural resonance of the voice coil, which is hung along the boundary between the dome portion and the edge portion of the diaphragm, thus improving the frequency response. Japanese Laid-Open Patent Publication No. 2006-287418 discloses a loudspeaker aiming at reducing the frequency response disturbance over the high range due to harmonic distortion, increasing the degree of freedom in adjusting the compliance in the edge damper, and ensuring an intended, unvaried compliance across the entire area around the diaphragm. In the loudspeaker, reinforcement ribs, being portions of a rugged structure, are formed along the outer periphery portion of the edge damper body, which is connected integrally with the outer periphery of a dome-shaped diaphragm in order to make uniform the compliance around the diaphragm and to improve the degree of freedom in adjusting the compliance around the diaphragm. In the dome-shaped diaphragm, on the other hand, reinforcement ribs, being grooves or ridges, extend radially with respect to the dome center, i.e., extend from near the dome center portion toward the outer periphery of the dome, in order to improve the rigidity.
International Publication WO2004/047487 discloses an electroacoustic transducer comprising a membrane with a middle area comprising stiffening grooves, wherein a membrane (20) has a membrane axis (5) and a middle area (50), a central cup-shaped depression (52) being provided around said membrane axis (5), which depression (52) preferably has a connecting channel (53), wherein the middle area (50) comprises groups of stiffening grooves (54, 55, 56, 57) which extend parallel to radial directions and of which a first group of long stiffening grooves (54, 55, 56) extends up to the depression (52), said connecting channel (53) issuing into two of the long stiffening grooves (55, 56), thus interconnecting these two long stiffening grooves (55, 56).
However, the conventional loudspeaker diaphragms and loudspeakers may not be sufficient for realizing both a further reduction in the thickness of a loudspeaker suitable for a portable electronic device and a countermeasure against peaks and dips in the frequency response over the high range. For example, when a concave dome-shaped depression is formed in a portion of a generally dome-like shape in order to reduce the overall height dimension of a loudspeaker diaphragm, the boundary between the concave dome-shaped depression and the convex dome-shaped portion acts as a node of a vibration mode to prevent a stable operation, wherein dips occur in the frequency response over the high range, thus failing to realize a desirable sound reproduction.